In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in an imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder, which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic and the necessary ink-repellency is provided by an initial application of a dampening (or “fountain”) solution to the plate prior to inking. The ink-adhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing member corresponding to each color is required. The original image is decomposed into imagewise patterns, or “separations,” that each reflects the contribution of the corresponding printable color. The positions of the printing members are coordinated so that the color components printed by the different members will be in register on the printed copies. Each printing member ordinarily is mounted on (or integral with) a “plate” cylinder, and the set of cylinders associated with a particular color on a press is usually referred to as a printing station.
In most conventional presses, the printing stations are arranged in a straight or “in-line” configuration. Each such station typically includes an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink (and, in wet systems, dampening) assemblies. The recording material is transferred among the print stations sequentially, each station applying a different ink color to the material to produce a composite multi-color image. Another configuration, described in U.S. Pat. No. 4,936,211, relies on a central impression cylinder that carries a sheet of recording material past each print station, eliminating the need for mechanical transfer of the medium to each print station. With either type of press, the recording medium can be supplied to the print stations in the form of cut sheets or a continuous “web” of material.
o circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers (e.g., cavity-type lasers). For example, U.S. Pat. Nos. 5,339,737; 5,351,617; 5,385,092; 5,822,345; and 5,990,925, the entirety of which are incorporated herein by reference, disclose ablative/sub-ablative recording systems that use laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from a diode to a printing surface and focused onto that surface (or, desirably, onto a layer most susceptible to laser ablation, which will generally lie beneath the surface layer). Other systems use laser energy to cause transfer of material from a donor to an acceptor sheet, to record non-ablatively, or as a pointwise alternative to overall exposure through a photomask or negative.
A challenge in designing laser-based imaging systems is achieving a beam having a high degree of symmetry and energy concentration while minimizing cost and equipment footprint. In general, the output should be circular in nature and feature a single dominant peak. The degree to which actual output approaches the ideal of a diffraction-limited source can be quantified, and this quantity used to assess the quality of the output. In particular, the widely used M factor relates beam resolution to the ideal of a diffraction-limited source (i.e., the beam quality factor. M2=1, where M2 =θD0 π/4wB, D0 is the diameter of the beam waist, θ is the beam divergence, and wB is the beam wavelength). The beam quality of a cavity laser, unfortunately, tends to vary with cavity length (for a given output power level)—the longer the cavity (and the larger the resulting laser assembly), the closer M2 will be to unity and, consequently, the better the beam. What is needed, therefore, is a design that combines relatively high beam qualities with relatively short cavity lengths.